Electrostatically oscillated device

ABSTRACT

An oscillator is oscillatable in a predetermined direction. First and second driving electrodes are secured to the base and apply an electrostatic force to the oscillator to make drive oscillation of the oscillator in the predetermined direction. At the time of the drive oscillation of the oscillator, a predetermined electric charge is accumulated in the oscillator, and electric charges of opposite polarities are alternately and periodically accumulated in the first and second driving electrodes, respectively, to exert an attractive force between the oscillator and a corresponding one of the first and second driving electrodes and also to exert a repulsive force between the oscillator and the other one of the first and second driving electrodes, and vice versa.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 11/075,708 filedon Mar. 10, 2005 now U.S. Pat. No. 7,159,462, and is based on andincorporates herein by reference Japanese Patent Application No.2004-70062 filed on Mar. 12, 2004 and Japanese Patent Application No.2004-70063 filed on Mar. 12, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrostatically oscillated device,such as an electrostatically oscillated angular velocity sensor or anelectrostatically oscillated actuator, which has an oscillator that isoscillated by electrostatic forces.

2. Description of Related Art

For instance, an electrostatically oscillated angular velocity sensor,which is fabricated by, for example, etching a semiconductor substrateto form a base, an oscillator and driving electrodes for driving theoscillator, has been proposed as the electrostatically oscillated device(see, for example, Japanese Unexamined Patent Publication No.2003-511684, which corresponds to U.S. Pat. No. 6,470,748, or JapaneseUnexamined Patent Publication No. 2001-91265, which corresponds to U.S.Pat. No. 6,450,033).

In the electrostatically oscillated angular velocity sensor, theoscillator is driven through the driving electrodes to generate driveoscillation in a predetermined direction. In this oscillated state, whenan angular velocity is applied, the oscillator is also oscillated in anorthogonal direction, which is orthogonal to the predetermined directionof the drive oscillation, to generate measurement oscillation due togeneration of a Coriolis force. Through measurement of this measurementoscillation in the orthogonal direction, the degree of the angularvelocity is determined.

Specifically, in the electrostatically oscillated angular velocitysensor, the oscillator is oscillated by the electrostatic forces. Morespecifically, flat electrodes or toothed electrodes are provided on leftand right sides, respectively, of the oscillator. A predeterminedvoltage is applied to the oscillator, and two voltages, which are ofopposite phases, are applied to the left and right driving electrodes,so that the oscillator is oscillated.

However, in the previously proposed electrostatically oscillated angularvelocity sensor, there is only a little drive force, which correspondsto a difference between an attractive force of the left drivingelectrode and an attractive force of the right driving electrode. Thus,the efficiency of the sensor is not high. This point will be morespecifically described with reference to FIG. 10.

FIG. 10 is a schematic plan view showing a structure of one previouslyproposed electrostatically oscillated angular velocity sensor, i.e., onepreviously proposed electrostatically oscillated device.

In the manufacturing of such an angular velocity sensor, asilicon-on-insulator (SOI) board, which includes two silicon plates thatare joined through an oxide film, is processed using known semiconductorprocessing technology.

An oscillator 30 is secured to a base 20 through driving bridges 33. Thedriving bridges 33 are resiliently deformable in an x-direction in FIG.10. Toothed driving electrodes 40, 41 are secured to the base 20. Thedriving electrodes 40, 41 apply electrostatic forces to the oscillator30 to drive the oscillator 30 and thereby to generate the driveoscillation of the oscillator 30 in the x-direction. The drivingelectrodes 40, 41 include first and second driving electrodes 40, 41. InFIG. 10, the first and second driving electrodes 40, 41 are provided onthe left and right sides, respectively, of the oscillator 30 to opposeeach other in the x-direction.

Furthermore, in FIG. 10, a sensing mass 32 is arranged in the center ofthe oscillator 30. The sensing mass 32 is connected to the rest of theoscillator 30 by sensing bridges 34, which are resiliently deformable ina y-direction. Two sensing electrodes 50 are secured to the base 20 attwo locations, respectively, which are opposed to the sensing mass 32.

In the electrostatically oscillated angular velocity sensor shown inFIG. 10, a predetermined voltage is applied to the oscillator 30, andtwo alternating voltages (drive signals), which are of opposite phases,are applied to the left and right driving electrodes 40, 41,respectively. As a result, the oscillator 30 is driven through thedriving bridges 33 to generate the drive oscillation of the oscillator30 in the x-direction.

Specifically, the predetermined voltage V0 is applied from a directcurrent (DC) power source 110 to the oscillator 30. Through use of analternating current (AC) power source 100 and an inverter 120, thealternating voltage V1 is applied to the first driving electrode 40, andthe alternating voltage V1′, which has the phase that is opposite to thephase of the alternating voltage V1, is applied to the second drivingelectrode 41.

Thus, the electrostatic attractive force F1 is exerted between the firstdriving electrode 40 and the oscillator 30, and the electrostaticattractive force F2 is exerted between the second driving electrode 41and the oscillator 30. The electrostatic force F1, which is exertedbetween the first driving electrode 40 and the oscillator 30, isexpressed by F1∝|V0−V1|. Similarly, the electrostatic force F2, which isexerted between the second driving electrode 41 and the oscillator 30,is expressed by F2∝|V0−V2|.

A difference (F1−F2) between the electrostatic attractive force F1 andthe electrostatic attractive force F2 is used as a drive force forgenerating the drive oscillation of the oscillator 30 in thex-direction.

In the above state where the oscillator 30 is driven to generate thedrive oscillation, when an angular velocity Ω is applied around a z-axisin FIG. 10, the Coriolis force is generated in the oscillator 30 in they-direction. Thus, the sensing mass 32 of the oscillator 30, which issupported by the sensing bridges 34, is oscillated in the y-direction bythe Coriolis force to produce measurement oscillation.

The capacitance between each sensing electrode 50 and the sensing mass32 changes due to the measurement oscillation. The change in thecapacitance is measured through the corresponding C/V converter 130 todetermine the degree of the angular velocity Ω.

When the oscillator 30 is driven in the above described manner togenerate the drive oscillation, the angular velocity Ω can be measured.However, as described above, only the small difference (F1−F2) betweenthe electrostatic attractive forces F1, F2 is used to drive theoscillator 30. Thus, the efficiency of the drive oscillation is nothigh.

The above disadvantage is not specific to the electrostaticallyoscillated angular velocity sensor but is commonly encountered when theefficiency of the drive oscillation, i.e., the amplitude of the driveoscillation needs to be increased in the electrostatically oscillateddevices, which has the above oscillator and driving electrodes.

Furthermore, the Coriolis force is proportional to the oscillating speedof the drive oscillation of the oscillator 30. Thus, the oscillatingspeed of the drive oscillation of the oscillator 30 needs to beincreased to increase the sensitivity of the angular velocity andthereby to accurately sense the angular velocity. To achieve this goal,the number of the electrode teeth of each driving electrode needs to beincreased to increase the drive force, i.e., the electrostatic force.For example, in the case of the sensor shown in FIG. 10, the number ofthe electrode teeth of each driving electrode 40, 41 should beincreased.

However, when the number of the electrode teeth of each drivingelectrode is simply increased, the size of the board of the sensor isdisadvantageously increased. To address this disadvantage, the inventorof the present invention has made a prototype electrostaticallyoscillated angular velocity sensor.

The prototype sensor of FIG. 11 is produced by forming two frames 131 inthe left and right parts, respectively, of the oscillator 30 of theprototype sensor of FIG. 10.

In FIG. 11, a portion of each driving electrode 140, 141, which issecured to a base 320, is placed inside the corresponding frame 131 toform a frame interior side securing portion 160. A toothed drivingelectrode portion 140 b, 141 b is provided to each frame interior sidesecuring portion 160, which is surrounded by the corresponding frame131.

That is, each driving electrode 140, 141 of the second prototype sensorhas the toothed first driving electrode part 140 a, 141 a and thetoothed second driving electrode part 140 b, 141 b. The first drivingelectrode part 140 a, 141 a is opposed to the corresponding outer sideof the oscillator 330, and the second driving electrode part 140 b, 141b is provided to the corresponding frame interior side securing portion160 and is opposed to the inner side of the corresponding frame 131.

When the oscillator 330 is formed to have the frames 131, and thetoothed driving electrode part 140 b, 141 b is additionally providedinside each frame 131, it is possible to increase the number of theelectrode teeth to increase the total effective electrode surface area,which aids in the drive oscillation of the oscillator 330. Due to theincrease of the number of the electrode teeth, which is made possible bythe additional second driving electrode parts 140 b, 141 b, theelectrostatic force, which is applied to the oscillator 330, should beincreased.

However, in the case of the structure shown in FIG. 11, an oppositeelectrostatic force, which is exerted in a direction opposite from thedriving electrostatic force used in generation of the drive oscillationof the oscillator 330, is exerted in each space (hereinafter, referredto as a back surface side space) 170. The back surface side space 170 isdefined between a back surface 160 a of each frame interior sidesecuring portion 160, which is opposite from the teeth of thecorresponding second driving electrode part 140 b, 141 b of the frameinterior side securing portion 160, and the inner side of the frame 131,which is opposed to the back surface 160 a.

Thus, although the number of the electrode teeth is increased, the driveforce is not increased in proportional to the number of the electrodeteeth, so that the oscillating speed of the drive oscillation of theoscillator 330 is not proportionally increased. The above disadvantageis not specific to the electrostatically oscillated angular velocitysensor but is commonly encountered when the drive force needs to beincreased in the electrostatically oscillated devices having theoscillator that includes the frame, which surrounds the toothedelectrode portion.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is anobjective of the present invention to provide an electrostaticallyoscillated device, which has an oscillator and driving electrodes andachieves an increased amplitude of oscillation of the oscillator. It isanother objective of the present invention to provide anelectrostatically oscillated device, which includes a base, anoscillator and driving electrodes and which provides an appropriatelyincreased drive force for oscillating the oscillator while minimizing anincrease in a size of the electrostatically oscillated device.

To address the above objectives of the present invention, there isprovided an electrostatically oscillated device, which includes a base,an oscillator and first and second driving electrodes. The oscillator ismovable relative to the base. Also, the oscillator is oscillatable in apredetermined direction. The first and second driving electrodes aresecured to the base and apply an electrostatic force to the oscillatorto make drive oscillation of the oscillator in the predetermineddirection. The first and second driving electrodes are arranged on firstand second sides, respectively, of the oscillator, which are opposed toeach other in the predetermined direction. At the time of the driveoscillation of the oscillator, a predetermined electric charge isaccumulated in the oscillator, and electric charges of oppositepolarities are alternately and periodically accumulated in the first andsecond driving electrodes, respectively, to exert an attractive forcebetween the oscillator and a corresponding one of the first and seconddriving electrodes and also to exert a repulsive force between theoscillator and the other one of the first and second driving electrodes.

To achieve the objectives of the present invention, there is alsoprovided an electrostatically oscillated device, which includes a base,an oscillator, first and second driving electrodes, at least one firstdriving electrode side dummy portion and at least one second drivingelectrode side dummy portion. The oscillator is movable relative to thebase. Also, the oscillator is oscillatable in a predetermined directionand includes first and second frames, which are arranged one afteranother in the predetermined direction. First and second drivingelectrodes are secured to the base and apply an electrostatic force tothe oscillator to make drive oscillation of the oscillator in thepredetermined direction. The first and second driving electrodes arearranged on first and second sides, respectively, of the oscillator,which are opposed to each other in the predetermined direction. Thefirst driving electrode includes a primary driving electrode portion, aframe interior side securing portion and a secondary driving electrodeportion. The primary driving electrode portion is opposed to a firstside outer peripheral portion of the oscillator in the predetermineddirection. The frame interior side securing portion is secured to thebase and extends from the primary driving electrode portion of the firstdriving electrode into the first frame. The secondary driving electrodeportion is provided to the frame interior side securing portion of thefirst driving electrode to oppose an inner peripheral portion of thefirst frame in the predetermined direction. The second driving electrodeincludes a primary driving electrode portion, a frame interior sidesecuring portion and a secondary driving electrode portion. The primarydriving electrode portion is opposed to a second side outer peripheralportion of the oscillator in the predetermined direction. The frameinterior side securing portion of the second driving electrode issecured to the base and extends from the primary driving electrodeportion of the second driving electrode into the second frame. Thesecondary driving electrode portion is provided to the frame interiorside securing portion of the second driving electrode to oppose an innerperipheral portion of the second frame in the predetermined direction.The at least one first driving electrode side dummy portion ispositioned inside the first frame between the first side outerperipheral portion of the oscillator and the secondary driving electrodeportion of the first driving electrode in the predetermined directionand has an electric potential that is in a floating state or is the sameas that of the first frame. The at least one second driving electrodeside dummy portion is positioned inside the second frame between thesecond side outer peripheral portion of the oscillator and the secondarydriving electrode portion of the second driving electrode in thepredetermined direction and has an electric potential that is in afloating state or is the same as that of the second frame.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features andadvantages thereof, will be best understood from the followingdescription, the appended claims and the accompanying drawings in which:

FIG. 1 is a schematic plan view of an electrostatically oscillatedangular velocity sensor, which serves as an electrostatically oscillateddevice, according to a first embodiment of the present invention;

FIG. 2 is a cross sectional view taken along line II-II in FIG. 1;

FIGS. 3A to 3C are diagrams, which respectively indicate cross sectionssimilar to that of FIG. 2 and show manufacturing steps of the angularvelocity sensor of the first embodiment;

FIGS. 4A to 4D are diagrams, which are similar to FIGS. 3A to 3C andshow manufacturing steps that follow the manufacturing steps of FIGS. 3Ato 3C;

FIG. 5 is a diagram showing a circuit structure of the angular velocitysensors of the first embodiment;

FIG. 6 is a schematic plan view of an angular velocity sensor accordingto a second embodiment of the present invention;

FIG. 7 is a cross sectional view taken along line VII-VII in FIG. 6;

FIGS. 8A to 8C are diagrams, which respectively indicate cross sectionssimilar to that of FIG. 7 and show manufacturing steps of the angularvelocity sensors of the second embodiment;

FIGS. 9A to 9D are diagrams, which are similar to FIGS. 8A to 8C andshow manufacturing steps that follow the manufacturing steps of FIGS. 8Ato 8C;

FIG. 10 is a schematic plan view of a previously proposedelectrostatically oscillated angular velocity sensor, which serves as anelectrostatically oscillated device; and

FIG. 11 is a schematic plan view of another previously proposedprototype angular velocity sensor.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

A first embodiment of the present invention will be described withreference to FIGS. 1 to 5. FIG. 1 is a plan view of an electrostaticallyoscillated angular velocity sensor S1, which serves as anelectrostatically oscillated device, according to the first embodimentof the present invention. FIG. 2 is a cross sectional view taken alongline II-II in FIG. 1.

With reference to FIG. 2, a board of the angular velocity sensor S1 is asilicon-on-insulator (SOI) board 10, which includes first and secondsilicon plates 11, 12 and an oxide film 13 interposed therebetween.

Trenches 12 a are formed in the second silicon plate 12 to define anoscillator 30, driving electrodes 40, 41, sensing electrodes 50 andbridges 33, 34 in an etching process.

Furthermore, the first silicon plate 11 and the oxide film 13 areeliminated by etching in a portion of the SOI board 10, whichcorresponds to the oscillator 30, so that an opening 14 is formed. Anouter peripheral portion of the first silicon plate 11 and of the oxidefilm 13 around the opening 14 is formed as a support arrangement, i.e.,a base 20.

The oscillator 30, which is arranged above the opening 14, is secured tothe base 20 through the driving bridges 33. In the present embodiment,the oscillator 30 is connected to the base 20 through the four drivingbridges 33.

Furthermore, with reference to FIG. 1, a sensing mass 32 is provided inthe center of the oscillator 30 and is connected to its adjacent partsof the oscillator 30 through the sensing bridges 34. In the presentembodiment, the sensing mass 32 is connected to the adjacent parts ofthe oscillator 30 through the four sensing bridges 34.

Here, with reference to FIG. 1, the driving bridges 33 are relativelyfreely deflectable in an x-direction and are limited from deflection ina y-direction. Thus, the driving bridges 33 allow oscillation of theoscillator 30 in the x-direction. In contrast, the sensing bridges 34are relatively freely deflectable in the y-direction and are limitedfrom deflection in the x-direction. Thus, the sensing bridges 34 allowsoscillation of the oscillator 30 in the y-direction.

Furthermore, in the second silicon plate 12, which is secured to thebase 20, the driving electrodes 40, 41 are formed on the left and rightsides of the oscillator 30 to oppose each other in the x-direction. Thedriving electrodes 40, 41 are provided to apply electrostatic forces tothe oscillator 30 to generate the drive oscillation of the oscillator 30in the x-direction.

The driving electrodes 40, 41 include first and second drivingelectrodes 40, 41. The first and second driving electrodes 40, 41 areprovided on the left and right sides, respectively, of the oscillator 30to oppose each other in the x-direction. In the present embodiment, eachof the first and second driving electrodes 40, 41 is formed as a tootheddriving electrode that has a plurality of teeth (i.e., a drivingelectrode portion having the teeth). The teeth of each of the first andsecond driving electrodes 40, 41 and teeth of a corresponding opposedtooth arrangement 30 a of the oscillator 30 are alternately arranged inthe y-direction.

The two sensing electrodes 50 are secured to the base 20 at twolocations, respectively, which are opposed to the sensing mass 32.Specifically, the sensing electrodes 50 are arranged on the opposedsides of the sensing mass 32 of the second silicon plate 12, which areopposed to each other in the y-direction.

The sensing electrodes 50 are provided to sense the oscillation(measurement oscillation) of the sensing mass 32 in the y-directiongenerated at the time of applying the angular velocity Ω around thez-axis, which is perpendicular to the x-direction and the y-direction,in the presence of the drive oscillation of the oscillator 30. Then, thesensing electrodes 50 output measurement signals, which correspond tothe sensed oscillation (the measurement oscillation) of the sensing mass32.

Here, pads (driving electrode side pads) 45, which are made of, forexample, aluminum, are provided to the driving electrodes 40, 41,respectively. Also, pads (sensing electrode side pads) 55, which aremade of, for example, aluminum, are provided to the sensing electrodes50, respectively. Each of the pads 45, 55 is electrically connected to acircuit (FIG. 5) through, for example, wire bonding.

Furthermore, at a securing portion of each driving bridge 33 to the base20, a pad (oscillator side pad) 35 is formed from, for example,aluminum. Each of the pads 35 is electrically connected to the circuit(FIG. 5) through, for example, wire bonding.

Next, the manufacturing method of the angular velocity sensor S1, whichis made of the silicon-on-insulator (SOI), will be described. FIGS. 3Ato 4D show the manufacturing method of the angular velocity sensor S1.Specifically, each of FIGS. 3A to 4D shows a cross section of acorresponding workpiece in a corresponding manufacturing step.

First, as shown in FIG. 3A, the SOI board 10 is prepared. The SOI board10 includes the first and second silicon plates 11, 12 and the oxidefilm 13. The first and second silicon plates 11, 12 are made of singlecrystal silicon. The oxide film 13 has a thickness of, for example, 1 μmand is interposed between the first silicon plate 11 and the secondsilicon plate 12.

Then, phosphorus or the like is diffused (N+ diffusion) into the entiresurface of the second silicon plate 12 at a high density to reduce thecontact resistance between the second silicon plate 12 and eachcorresponding aluminum pad 35, 45, 55 (only the pads 45 are depicted inthe drawing).

Next, the respective pads 35, 45, 55 are formed by vapor depositingaluminum of, for example, 1 μm thickness onto the surface (the secondsilicon plate 12) of the SOI board 10 and then by photo-etching thealuminum.

Next, as shown in FIG. 3B, the back surface (the first silicon plate 11)of the SOI board 10 is ground and is polished through back polishing toa predetermined thickness (e.g., 300 μm), so that the back surface (thefirst silicon plate 11) of the SOI board 10 is mirror finished.

Thereafter, as shown in FIG. 3C, a plasma SiN film 300 of, for example,0.5 μm, is deposited onto the back surface (the first silicon plate 11)of the SOI board 10 to form a photo pattern. Then, the plasma SiN film300 is etched to form an opening in a predetermined area of the plasmaSiN film 300.

Next, with reference to FIG. 4A, a pattern, which defines the oscillator30, the driving electrodes 40, 41, the sensing electrodes 50 and thebridges 33, 34, is formed on the surface of the second silicon plate 12.Then, the trenches 12 a, which reach the oxide film 13, are formedvertically by dry etching.

Then, as shown in FIG. 4B, the first silicon plate 11 is deeply etchedin KOH solution while the pattern formed in the plasma SiN film 300 isused as a mask.

At this time, when the etching proceeds to the oxide film 13, the oxidefilm 13 may be destroyed by the fluid pressure of the etching solution.Thus, the etching time should be carefully controlled to end in a mannerthat leave the silicon of 10 μm in the first silicon plate 11 to limitthe destruction of the oxide film 13.

Next, as shown in FIG. 4C, the silicon (Si), which is left in the stepof FIG. 4B, is removed by plasma dry etching. At this time, the plasmaSiN film 300, which is present on the back surface of the SOI board 10,is simultaneously removed.

Finally, as shown in FIG. 4D, the oxide film 13 is removed from thecorresponding part by dry etching, so that the oscillator 30 is formed.In this way, the manufacturing of the angular velocity sensor S1 iscompleted. Thereafter, each of the pads 35, 45, 55 is electricallyconnected to the circuit (FIG. 5) by, for example, wire bonding.

Next, operation of the angular velocity sensor S1 will be described.FIG. 5 shows a circuit structure of the angular velocity sensor S1,which includes the above described circuit.

As shown in FIG. 5, the circuit includes an alternating current (AC)power surface 100, a direct current (DC) power source 110, an inverter120 and capacitance-to-voltage (C/V) converters 130. The circuit can bean integrated circuit, which is formed integrally on the SOI board 10having the angular velocity sensor S1. Alternatively, the circuit can bea separate circuit, which is separated from the SOI board 10.

An oscillator side capacitor 200 is connected between the DC powersource 110 and the oscillator 30 and is electrically connected to theoscillator 30. The oscillator side capacitor 200 is provided toaccumulate electric charge in the oscillator 30.

Furthermore, a first driving electrode side capacitor 210 is connectedbetween the AC power source 100 and the first driving electrode 40. Thefirst driving electrode side capacitor 210 is electrically connected tothe first driving electrode 40. The first driving electrode sidecapacitor 210 is provided to accumulate electric charge in the firstdriving electrode 40.

Furthermore, a second driving electrode side capacitor 220 is connectedbetween the inverter 120 and the second driving electrode 41. The seconddriving electrode side capacitor 220 is electrically connected to thesecond driving electrode 41. The second driving electrode side capacitor220 is provided to accumulate electric charge in the second drivingelectrode 41.

The capacitance between the first driving electrode 40 and theoscillator 30 (specifically, the left tooth arrangement 30 a of theoscillator 30 in FIG. 5) is set to be equal to the capacitance betweenthe second driving electrode 41 and the oscillator 30 (specifically, theright tooth arrangement 30 a of the oscillator 30 in FIG. 5).

Furthermore, the capacitance of the first driving electrode sidecapacitor 210 is set to be equal to the capacitance of the seconddriving electrode side capacitor 220. Here, for ease of explanation, thecapacitance of the oscillator side capacitor 200 is denoted as C0, andeach of the capacitance of the first driving electrode side capacitor210 and the capacitance of the second driving electrode side capacitor220 is denoted as C1.

Each of the capacitance C0 of the oscillator side capacitor 200, thecapacitance C1 of the first driving electrode side capacitor 210 and thecapacitance C1 of the second driving electrode side capacitor 220 isgreater than the capacitance between the first driving electrode 40 andthe oscillator 30.

For example, it is desirable to set each of the capacitance C0 and thecapacitance C1 to be at least ten times greater than the capacitancebetween the first driving electrode 40 and the oscillator 30 toaccumulate sufficient electric charge in each corresponding component.

Each capacitor 200, 210, 220 can be formed integrally as a part of anintegral circuit, which is integrated in the SOI board 10.Alternatively, each capacitor 200, 210, 220 can be formed as a part of aseparate circuit board, which is formed separately from the SOI board10. Furthermore, each capacitor 200, 210, 220 can be formed by asemiconductor manufacturing process or can be formed as a parasiticcapacitor of the SOI board 10. Also, each capacitor 200, 210, 220 can beformed as an external discrete component, such as an electrolyticcapacitor.

In the above circuit structure, a predetermined electric potential V0 isapplied from the DC power source 110 to the oscillator 30 when theoscillator 30 is driven to oscillate in the x-direction. An electriccharge Q0, which is accumulated in the oscillator side capacitor 200, isexpressed as Q0=C0·V0, and a predetermined electric charge (−Q0) isaccumulated in the oscillator 30.

In the above state where the predetermined electric charge (−Q0) isaccumulated in the oscillator 30, the alternating current, which has thesine waveform, is outputted from the AC power source 100. At this time,the voltage, which has the phase opposite from that of the first drivingelectrode 40, is applied to the second driving electrode 41.

In an exemplary case where the positive voltage V1 is applied from theAC power source 100 to the first driving electrode side capacitor 210,the electric charge Q1, which is accumulated in the first drivingelectrode side capacitor 210, is expressed as Q1=C1·V1. The electriccharge (+Q1), which has the polarity opposite from that of theoscillator 30, is accumulated in the first driving electrode 40.

At this time, the voltage V1, which has the phase opposite from that ofthe first driving electrode side capacitor 210, is applied from theinverter 120 to the second driving electrode side capacitor 220. Thus,the electric charge, which is accumulated in the second drivingelectrode side capacitor 220, is expressed as −Q1. In this way, theelectric charge (−Q1), which has the same polarity as that of theoscillator 30, is accumulated in the second driving electrode 41.

In the above circuit, while the predetermined electric charge isaccumulated in the oscillator 30, the electric charges, which have theopposite polarities, respectively, can be alternately and periodicallyaccumulated in the first driving electrode 40 and the second drivingelectrode 41.

Specifically, the electric charges, which have the opposite polarities,are alternately applied to the driving electrodes 40, 41 in thefollowing manner. That is, in one state where the electric charge, whichhas the same polarity as that of the oscillator 30, is accumulated inthe first driving electrode 40, the electric charge, which has thepolarity opposite from that of the oscillator 30, is accumulated in thesecond driving electrode 41. Furthermore, in the other state where theelectric charge, which has the polarity opposite from that of theoscillator 30, is accumulated in the first driving electrode 40, theelectric charge, which is the same as that of the oscillator 30, isaccumulated in the second driving electrode 41.

Thus, at the time of drive oscillation of the oscillator 30, anattractive force is exerted between one of the first and second drivingelectrodes 40, 41 and the corresponding tooth arrangement 30 a of theoscillator 30, and a repulsive force is exerted between the other one ofthe first and second driving electrodes 40, 41 and the correspondingtooth arrangement 30 a of the oscillator 30, and vice versa.

When the electric charges are alternately applied from the AC powersource 100 to the driving electrodes 40, 41, the oscillator 30 is drivento make the drive oscillation at the frequency of the alternatingelectric current in the x-direction by the above-described attractiveforce and the repulsive force.

In the above operational stage where the oscillator 30 is driven togenerate the drive oscillation, when the angular velocity Ω is appliedaround the z-axis, the Coriolis force is generated in the oscillator 30in the y-direction. Thus, the sensing mass 32 of the oscillator 30 isoscillated in the y-direction by the Coriolis force to produce themeasurement oscillation.

The capacitance between each sensing electrode 50 and the sensing mass32 changes due to the measurement oscillation. The change in thecapacitance is measured through the corresponding C/V converter 130 todetermine the degree of the angular velocity Ω.

According to the present embodiment, there is provided the angularvelocity sensor S1, which serves as the electrostatically oscillateddevice and which includes the base 20, the oscillator 30 and the firstand second driving electrodes 40, 41. The oscillator 30 is arranged tobe movable relative to the base 20 and is oscillatable in thepredetermined direction, i.e., in the x-direction. The first and seconddriving electrodes 40, 41 are provided to apply the electrostatic forcesto the oscillator 30, which is secured to the base 20, to generate thedrive oscillation of the oscillator 30 in the x-direction. The firstdriving electrode 40 is provided on one side of the oscillator 30, andthe second driving electrode 41 is provided on the other side of theoscillator 30. The angular velocity sensor S1 provides the followingadvantages.

That is, there is provided the angular velocity sensor S1, in which theelectric charges, which have the opposite polarities, respectively, arealternately and periodically accumulated in the first driving electrode40 and the second driving electrode 41 in the following manner. That is,while the predetermined electric charge is accumulated in the oscillator30 at the time of generating the drive oscillation of the oscillator 30,the attractive force is exerted between one of the first and seconddriving electrodes 40, 41 and the oscillator 30, and the repulsive forceis exerted between the other one of the first and second drivingelectrodes 40, 41 and the oscillator 30, and vice versa.

As discussed above, the electric charges, which have the oppositepolarities, respectively, are alternately accumulated in the first andsecond driving electrodes 40, 41 while the predetermined electric chargeis accumulated in the oscillator 30. Thus, throughout the time ofgenerating the drive oscillation of the oscillator 30, the attractiveforce is exerted between one of the first and second driving electrodes40, 41 and the oscillator 30, and the repulsive force is exerted betweenthe other one of the first and second driving electrodes 40, 41 and theoscillator 30, and vice versa.

Therefore, according to the present embodiment, the drive force forgenerating the drive oscillation of the oscillator 30 is the sum ofattractive force, which is exerted between one of the driving electrodes40, 41 and the oscillator 30, and the repulsive force, which is exertedbetween the other one of the driving electrodes 40, 41 and theoscillator 30.

That is, in the previously proposed sensor, the difference between theelectrostatic attractive force of one of the driving electrodes and theelectrostatic attractive force of the other one of the drivingelectrodes is used as the drive force for oscillating the oscillator. Incontrast, according to the present embodiment, the drive force is atleast two times greater than that of the previously proposed sensor.

As discussed above, in the present embodiment, although the structuresof the electrodes are the same as those of the previously proposedsensor, the voltage is not simply applied to the electrodes. Instead, inthe present embodiment, while predetermined electric charge isaccumulated in the oscillator 30, the electric charge, which has thepolarity opposite from that of the oscillator 30, is applied to one ofthe driving electrodes 40, 41, and the electric charge, which has thesame polarity as that of the oscillator 30, is applied to the other oneof the driving electrodes 40, 41, and vice versa. In this way, theelectrostatic attractive force and the electrostatic repulsive force areutilized to apply the electrostatic forces to the oscillator 30.

Thus, according to the present embodiment, in the angular velocitysensor S1, which serves as the electrostatically oscillated device andhas the driving electrodes 40, 41 on the opposed sides of the oscillator30, which are opposed in the direction of the drive oscillation of theoscillator 30, i.e., in the x-direction, the amplitude of theoscillation of the oscillator 30 is increased at the same power sourcevoltage, which is the same as that of the previously proposed sensor.

Also, in the present embodiment, the oscillator side capacitor 200,which is used to accumulate the electric charge in the oscillator 30, iselectrically connected to the oscillator 30. Furthermore, the firstdriving electrode side capacitor 210, which is used to accumulate theelectric charge in the first driving electrode 40, is electricallyconnected to the first driving electrode 40. In addition, the seconddriving electrode side capacitor 220, which is used to accumulate theelectric charge in the second driving electrode 41, is electricallyconnected to the second driving electrode 41.

With this construction, the accumulation of the electric charge in eachcorresponding component is appropriately accomplished in the presentembodiment.

In the first embodiment, the present invention is implemented in theangular velocity sensor. However, the present invention can be appliedto, for example, an electrostatically oscillated actuator.

Specifically, the present invention can be implemented in anyelectrostatically oscillated device, which includes the oscillator andthe first and second driving electrodes. In this device, the oscillatoris arranged to be movable relative to the base and is oscillatable inthe predetermined direction. Furthermore, the first and second drivingelectrodes are provided to apply the electrostatic forces to theoscillator, which is secured to the base, to generate the driveoscillation of the oscillator in the predetermined direction. Here, thefirst driving electrode is provided on the one side of the oscillator,and the second driving electrode is provided on the other side of theoscillator.

Second Embodiment

A second embodiment of the present invention will be described withreference to the accompanying drawings. FIG. 6 is a schematic plan viewof an electrostatically oscillated angular velocity sensor S101, whichserves as an electrostatically oscillated device, according to theembodiment of the present invention. FIG. 7 is a cross sectional viewtaken along line VII-VII in FIG. 6.

With reference to FIG. 7, a board of the angular velocity sensor S101 isa silicon-on-insulator (SOI) board 310, which includes first and secondsilicon plates 111, 112 and an oxide film 113 interposed therebetween.

Trenches 112 a are formed on the second silicon plate 112 to define anoscillator 330, driving electrodes 140, 141, sensing electrodes 150 andbridges 133, 134 in an etching process.

Furthermore, the first silicon plate 111 and the oxide film 113 areeliminated by etching in a portion of the SOI board 310, whichcorresponds to the oscillator 330, so that an opening 114 is formed. Anouter peripheral portion of the first silicon plate 111 and of the oxidefilm 113 around the opening 114 is formed as a support arrangement,i.e., a base 320.

The oscillator 330 includes two frames 131 and a sensing mass 132. Theframes 131 are arranged on the left and right sides, respectively, ofthe first silicon plate 112 in the x-direction. Each frame 131 is formedto have a frame shape in a plan view of the frame 131. The sensing mass132 is arranged between the frames 131 and is formed to have arectangular shape in a plan view of the sensing mass 132.

In the present embodiment, the oscillator 330 is connected to the base320 through the four driving bridges 133. Also, in the presentembodiment, the sensing mass 132 is connected to the frames 131 throughthe four sensing bridges 134.

Here, the driving bridges 133 are relatively freely deflectable in thex-direction and are limited from deflection in the y-direction in FIG.6. Thus, the driving bridges 133 allow oscillation of the oscillator 330in the x-direction. In contrast, the sensing bridges 134 are relativelyfreely deflectable in the y-direction and are limited from deflection inthe x-direction. Thus, the sensing bridges 134 allow oscillation of theoscillator 330 in the y-direction.

Furthermore, in the second silicon plate 112, which is secured to thebase 320, the driving electrodes 140, 141 are formed on the left andright sides of the frames 131 of the oscillator 330 to oppose each otherin the x-direction.

The driving electrodes 140, 141 are provided to apply the electrostaticforce to the oscillator 330 to generate the drive oscillation of theoscillator 330 in the x-direction. The driving electrode 140, which islocated on the left side in FIG. 6 will hereinafter be referred to as aleft driving electrode (or a first driving electrode) 140. Also, thedriving electrode 141, which is located on the right side in FIG. 6,will be hereinafter referred to as a right driving electrode (or asecond driving electrode) 141.

The left driving electrode 140 includes primary and secondary drivingelectrode portions 140 a, 140 b. The primary driving electrode portion140 a of the left driving electrode 140 is opposed to a left outerperipheral portion (a left outer tooth arrangement 330 a having aplurality of teeth) of the oscillator 330, and the secondary drivingelectrode portion 140 b of the left driving electrode 140 is opposed toan inner peripheral portion (a left inner tooth arrangement 330 b havinga plurality of teeth) of the left frame 131 of the oscillator 330.Similarly, the right driving electrode 141 includes primary andsecondary driving electrode portions 141 a, 141 b. The primary drivingelectrode portion 141 a of the right driving electrode 141 is opposed toa right outer peripheral portion (a right outer tooth arrangement 330 ahaving a plurality of teeth) of the oscillator 330, and the secondarydriving electrode portion 141 b of the right driving electrode 141 isopposed to an inner peripheral portion (a right inner tooth arrangement330 b having a plurality of teeth) of the right frame 131 of theoscillator 330.

The secondary driving electrode portion 140 b of the left drivingelectrode 140 is provided in a frame interior side securing portion 160of the left driving electrode 140, which is a part of the second siliconplate 112 secured to the base 320 and which is arranged inside the leftframe 131. Similarly, the secondary driving electrode portion 141 b ofthe right driving electrode 141 is provided in a frame interior sidesecuring portion 160 of the right driving electrode 141, which is a partof the second silicon plate 112 secured to the base 320 and which isarranged inside the right frame 131.

A portion of each frame 131 is cut through the frame 131 in thex-direction to form an opening 131 a, and the frame interior sidesecuring portion 160 is formed into a generally T-shaped body. TheT-shaped body of the frame interior side securing portion 160 extendsfrom a securing portion of the T-shaped body, which is secured to thebase 320, and is received in the interior of the frame 131 through theopening 131 a.

In the present embodiment, each of the primary and secondary drivingelectrode portions 140 a, 140 b, 141 a, 141 b is formed as a tootheddriving electrode portion that has a plurality of teeth. The teeth ofeach of the primary and secondary driving electrode portions 140 a, 140b, 141 a, 141 b and the teeth of a corresponding opposed one of thetooth arrangements 330 a, 330 b of the oscillator 330 are alternatelyarranged in the y-direction in FIG. 6.

Furthermore, the sensing electrodes 150 are arranged on the opposedsides of the sensing mass 132 of the second silicon plate 112, which areopposed to each other in the y-direction. The sensing electrodes 150 areprovided to sense the oscillation (measurement oscillation) of thesensing mass 132 in the y-direction generated at the time of applyingthe angular velocity Ω around the z-axis, which is perpendicular to thex-direction and the y-direction, in the presence of the driveoscillation of the oscillator 330. Then, the sensing electrodes 150output measurement signals, which correspond to the sensed oscillation(the measurement oscillation) of the sensing mass 132.

Here, pads (driving electrode side pads) 145, which are made of, forexample, aluminum, are provided to the driving electrodes 140, 141.Also, pads (sensing electrode side pads) 155, which are made of, forexample, aluminum, are provided to the sensing electrodes 150. Each ofthe pads 145, 155 are electrically connected to a circuit (not shown butsimilar to that of FIG. 5) through, for example, wire bonding.

Furthermore, at a securing portion of each driving bridge 133 to thebase 320, a pad (oscillator side pad) 135 is formed, from, for example,aluminum. Each of the pads 135 is electrically connected to the circuitthrough, for example, wire bonding.

Furthermore, in the present embodiment, a dummy portion 180 is providedin each corresponding space, i.e., each back surface side space 170. Theback surface side space 170 is defined between a back surface portion160 a of the corresponding frame interior side securing portion 160,which is opposite from the teeth of the secondary driving electrodeportion 140 b, 141 b, and an opposed inner peripheral portion of thecorresponding frame 131, which is opposed to the back surface portion160 a. The electric potential of each dummy portion 180 is in a floatingstate or is the same as the electric potential of the correspondingframe 131 (the oscillator 330). More specifically, in the present case,two dummy portions (first driving electrode side dummy portions) 180 arepositioned inside the left frame (first frame) between the left side(first side) outer peripheral portion of the oscillator 330 and thesecondary driving electrode portion 140 b of the left driving electrode(the first driving electrode) 140 in FIG. 6. Also, two dummy portions(second driving electrode side dummy portions) 180 are positioned insidethe right frame (second frame) between the right side (second side)outer peripheral portion of the oscillator 330 and the secondary drivingelectrode portion 141 b of the right driving electrode (the seconddriving electrode) 141 in FIG. 6.

Each dummy portion 180 can be formed as a part of the second siliconplate 112. Alternatively, each dummy portion 180 can be formed as aseparate component, such as a separate semiconductor, which is formedseparately from the second silicon plate 112. However, it is desirableto secure each dummy portion 180 to the base 320. The method forsecuring each dummy portion 180 to the base 320 can be one, which isknow to or which is obvious to a person skilled in the art.

For example, a dielectric member can be interposed between each dummyportion 180 and a portion of the second silicon plate 112, which issupported by the base 320 and is electrically separated from theoscillator 330 and each electrode 140, 141, 150, so that each dummyportion 180 is supported by the base 320 through the dielectric member.

Alternatively, each dummy portion 180 can be secured to an appropriateportion of the frame interior side securing portion 160 through adielectric joining member. The pattern of the trenches 112 a, whichdefine each corresponding component shown in FIG. 6, should beunderstood as the exemplary pattern of the trenches 112 a. For example,the pattern of the trenches 112 a can be changed to make each dummyportion 180 supported by the base 320 only at one longitudinal end ofthe dummy portion 180 and thereby to make each dummy portion 180cantilevered.

However, it should be noted that each dummy portion 180 can be mademovable within a limited range without contacting the correspondingframe 131 of the oscillator 330 and the corresponding frame interiorside securing portion 160.

When the electric potential of the dummy portion 180 needs to be made asthe same as that of the corresponding frame 131, i.e., of the oscillator330, an electrode, which is electrically connected to the dummy portion180 may be provided to an appropriate location to apply the same voltageas that of the oscillator 330 to the dummy portion 180 through thiselectrode.

Next, the manufacturing method of the angular velocity sensor S101,which is made of the silicon-on-insulator (SOI), will be described.FIGS. 8A to 9D show the manufacturing method of the angular velocitysensor S101. Specifically, each of FIGS. 8A to 9D shows a cross sectionof a corresponding workpiece, which is similar to that of FIG. 7, in acorresponding manufacturing step.

First, as shown in FIG. 8A, the SOI board 310 is prepared. The SOI board310 includes the first and second silicon plates 111, 112 and the oxidefilm 113. The first and second silicon plates 111, 112 are made ofsingle crystal silicon. The oxide film 113 has a thickness of, forexample, 1 μm and is interposed between the first silicon plate 111 andthe second silicon plate 112.

Then, phosphorus or the like is diffused (N+ diffusion) into the entiresurface of the second silicon plate 112 at a high density to reduce thecontact resistance between the second silicon plate 112 and eachcorresponding aluminum pad 135, 145, 155 (only the pads 145 are depictedin the drawing).

Next, the respective pads 135, 145, 155 are formed by vapor depositingaluminum of, for example, 1 μm thickness, onto the surface (the secondsilicon plate 112) of the SOI board 310 and then by photo-etching thealuminum.

Next, as shown in FIG. 8B, the back surface (the first silicon plate111) of the SOI board 310 is ground and is polished through backpolishing to a predetermined thickness (e.g., 300 μm), so that the backsurface (the first silicon plate 111) of the SOI board 310 is mirrorfinished.

Next, as shown in FIG. 8C, a plasma SiN film 400 of, for example, 0.5μm, is deposited onto the back surface (the first silicon plate 111) ofthe SOI board 310 to form a photo pattern. Then, the plasma SiN film 400is etched to form an opening in a predetermined area of the plasma SiNfilm 400.

Next, with reference to FIG. 9A, a pattern, which defines the oscillator330, the driving electrodes 140, 141, the sensing electrodes 150, thebridges 133, 134 and the dummy portions 180 of the back surface sidespaces 170, is formed on the surface of the second silicon plate 112.Then, the trenches 112 a, which reach the oxide film 113, are formedvertically by dry etching.

Then, as shown in FIG. 9B, the first silicon plate 111 is deeply etchedin KOH solution while the pattern formed in the plasma SiN film 400 isused as a mask.

At this time, when the etching proceeds to the oxide film 113, the oxidefilm 113 is destroyed by the fluid pressure of the etching solution.Thus, the etching time should be carefully controlled to end in a mannerthat leave the silicon of 10 μm in the first silicon plate 111 to limitthe destruction of the oxide film 113.

Next, as shown in FIG. 9C, Si, which is left in the step of FIG. 9B, isremoved by plasma dry etching. At this time, the plasma SiN film 400,which is present on the back surface of the SOI board 310, issimultaneously removed.

Finally, as shown in FIG. 9D, the oxide film 113 is removed by dryetching, so that the oscillator 330 is formed. Each dummy portion 180needs to be secured to the base 320 before the removal of the oxidationfilm 113. In this way, the manufacturing of the angular velocity sensorS101 is completed. Thereafter, each of the pads 135, 145, 155 iselectrically connected to the above circuit by, for example, wirebonding.

Next, operation of the angular velocity sensor S101 will be described.The predetermined voltage is applied from the above circuit to theoscillator 330 through the oscillator side pad 135 in a manner similarto the one discussed in the first embodiment with reference to FIG. 5.Also, the two alternating voltages (drive signals), which are ofopposite phases, are applied to the left and right driving electrodes140, 141, respectively, of FIG. 6 in a manner similar to the onediscussed in the first embodiment with reference to FIG. 5.

In this way, the electrostatic force (electrostatic attractive force) isgenerated between each corresponding tooth arrangement 330 a, 330 b ofthe oscillator 330 and the opposed driving electrode 140, 141, so thatthe entire oscillator 330 is driven through the driving bridges 133 tomake the drive oscillation in the x-direction.

In the above state where the oscillator 330 is driven to generate thedrive oscillation, when the angular velocity Ω is applied around thez-axis, the Coriolis force is generated in the oscillator 330 in they-direction. Thus, the sensing mass 132 of the oscillator 330 isoscillated in the y-direction by the Coriolis force to produce themeasurement oscillation.

Due to the measurement oscillation, the capacitance between each sensingelectrode 150 and the sensing mass 132 changes. Through measurement of achange in the capacitance between each sensing electrode 150 and thesensing mass 132, the angular velocity Ω can be determined.

According to the present embodiment, there is provided the angularvelocity sensor S101, which serves as the electrostatically oscillateddevice and which includes the base 320, the oscillator 330 and thedriving electrodes 140, 141. The oscillator 330 is arranged to bemovable relative to the base 320 and is oscillatable in thepredetermined direction, i.e., in the x-direction. The drivingelectrodes 140, 141 are provided to apply the electrostatic force to theoscillator 330, which is secured to the base 320, to generate the driveoscillation of the oscillator 330 in the x-direction. The angularvelocity sensor S101 provides the following advantages.

The oscillator 330 has the frames 131, each of which has the frame shapein the plan view of the frame 131. Each frame interior side securingportion 160 is placed inside the corresponding frame 131 and is securedto the base 320.

Each driving electrode 140, 141 has the toothed primary drivingelectrode portion 140 a, 141 a and the toothed secondary drivingelectrode portion 140 b, 141 b. The primary driving electrode portion140 a, 141 a is opposed to the corresponding outer side of theoscillator 330 in the x-direction, and the secondary driving electrodeportion 140 b, 141 b is provided to the corresponding frame interiorside securing portion 160 and is opposed to the inner peripheral side ofthe corresponding frame 131 in the x-direction.

Furthermore, the dummy portion 180 is provided in each correspondingspace, which is defined between the back surface portion 160 a of thecorresponding frame interior side securing portion 160, which isopposite from the teeth of the secondary driving electrode portion 140b, 141 b, and an opposed inner peripheral portion of the correspondingframe 131, which is opposed to the back surface portion 160 a in thex-direction. The electric potential of each dummy portion 180 is in thefloating state or is the same as the electric potential of the frame131.

In the angular velocity sensor S101 of the present embodiment, theoscillator 330 has the frames 131, in each of which the secondarydriving electrode portion 140 b, 141 b is provided. Thus, while anincrease in the size of the angular velocity sensor S101 is limited, theeffective surface area of each driving electrode, which aids in thedrive oscillation of the oscillator 330, is increased.

The dummy portions 180 are provided in the back surface side spaces 170and have the electric potential, which is in the floating state or isthe same as that of the frames 131. Thus, the electrostatic force ineach back surface side space 170 is reduced by the corresponding dummyportion 180.

That is, the back surface portion 160 a of each frame interior sidesecuring portion 160, which is opposite from the teeth of the secondarydriving electrode portion 140 b, 141 b of the frame interior sidesecuring portion 160, does not directly face the opposed innerperipheral portion of the frame 131 due to the positioning of the dummyportions 180 therebetween. Thus, the electrostatic force, which isapplied from the back surface 160 a and interferes with the driveoscillation, has the less effect on the frame 131.

Therefore, according to the present embodiment, in the angular velocitysensor, which serves as the electrostatically oscillated device and isfabricated by etching the board to have the base 320, the oscillator 330and the driving electrodes 140, 141, the drive force can beappropriately increased without substantially increasing the size of theangular velocity sensor.

In the present embodiment, it is desirable that the electric potentialof each dummy portion 180 is in the same state as that of the oscillator330. Since the electrostatic force is not exerted between each dummyportion 180 and the frame 131, the application of the undesirableelectrostatic force to the oscillator 330 in the direction, which isopposite from the effective electrostatic force for generating the driveoscillation of the oscillator 330, can be appropriately limited.

The second embodiment can be modified as follows.

In the above angular velocity sensor, the oscillator 330 and the othercorresponding components are formed by the back surface processing.Alternatively, the oscillator 330 and other corresponding components canbe formed by the front surface processing, in which sacrificial etchingof the oxide film 113 of the SOI board 310 may be performed, or thetrench etching and the side etching may be performed from the frontsurface side of the SOI board 310 to form the oscillator 330 and thecorresponding components. In this case, each dummy portion 180 issupported by the base 320 by performing the etching in a manner thatleaves the underneath oxide film 113, which is positioned underneath thedummy portion 180.

Each dummy portion 180 can be supported by a separate correspondingstructure, which is located below the SOI board 110 of the angularvelocity sensor S101 or below the second silicon plate 112 and supportsthe dummy portion 180. Alternatively, each dummy portion 180 can bemechanically secured to and electrically insulated from the adjacentdriving electrode 140, 141 or the oscillator 330. In such a case, thecorresponding electrical potential can be applied to each dummy portion180 through a separate wiring, which is separated from the adjacentdriving electrode 140, 141 and the oscillator 330.

In the above embodiment, the present invention is implemented in theangular velocity sensor. However, the present invention can beimplemented in, for example, an electrostatically oscillated actuator.

Specifically, the present invention can be implemented in theelectrostatically oscillated device, which has the base, the oscillatorand the driving electrodes, each of which has the driving electrodeportion received in the corresponding frame of the oscillator.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention in its broader terms is therefore notlimited to the specific details, representative apparatus, andillustrative examples shown and described.

1. An electrostatically oscillated device comprising: a base; anoscillator that is movable relative to the base, wherein the oscillatoris oscillatable in a predetermined direction and includes first andsecond frames, which are arranged one after another in the predetermineddirection; first and second driving electrodes that are secured to thebase and apply an electrostatic force to the oscillator to make driveoscillation of the oscillator in the predetermined direction, wherein:the first and second driving electrodes are arranged on first and secondsides, respectively, of the oscillator, which are opposed to each otherin the predetermined direction; the first driving electrode includes: aprimary driving electrode portion that is opposed to a first side outerperipheral portion of the oscillator in the predetermined direction; aframe interior side securing portion that is secured to the base andextends from the primary driving electrode portion of the first drivingelectrode into the first frame; and a secondary driving electrodeportion that is provided to the frame interior side securing portion ofthe first driving electrode to oppose an inner peripheral portion of thefirst frame in the predetermined direction; the second driving electrodeincludes: a primary driving electrode portion that is opposed to asecond side outer peripheral portion of the oscillator in thepredetermined direction; a frame interior side securing portion that issecured to the base and extends from the primary driving electrodeportion of the second driving electrode into the second frame; and asecondary driving electrode portion that is provided to the frameinterior side securing portion of the second driving electrode to opposean inner peripheral portion of the second frame in the predetermineddirection; at least one first driving electrode side dummy portion thatis positional inside the first frame between the first side outerperipheral portion of the oscillator and the secondary driving electrodeportion of the first driving electrode in the predetermined directionand has an electric potential that is in a floating state or is the sameas that of the first frame; and at least one second driving electrodeside dummy portion that is positional inside the second frame betweenthe second side outer peripheral portion of the oscillator and thesecondary driving electrode portion of the second driving electrode inthe predetermined direction and has an electric potential that is in afloating state or is the same as that of the second frame.
 2. Theelectrostatically oscillated device according to claim 1, wherein: theelectric potential of the at least one first driving electrode sidedummy portion is the same as that of the first frame; the electricpotential of the at least one second driving electrode side dummyportion is the same as that of the second frame; and the electricpotential of the first frame is the same as the electric potential ofthe second frame.
 3. The electrostatically oscillated device accordingto claim 1, wherein: the first frame has an opening, through which theframe interior side securing portion of the first driving electrodeextends into an interior of the first frame; and the second frame has anopening, through which the frame interior side securing portion of thesecond driving electrode extends into an interior of the second frame.